In image take apparatuses such as a digital camera and the like, practical use of a shape memory alloy actuator (=shape memory alloy dive device) that performs a lens movement by using a shape memory alloy is studied. A shape memory alloy shrinks when heated by a voltage or a current applied thereto. Besides, a shape memory alloy stretches by radiating heat. Accordingly, by connecting a movable portion such as a lens unit or the like to one end of a shape memory alloy, applying a voltage or a current to it, and performing servo control of the amount of a flowing current (hereinafter, called “drive control value”), it is possible to perform focusing and the like.
In a shape memory alloy actuator that is controlled in displacement by a servo as described above, to raise response, a loop gain is set in such a way that a drive control value in a transient period of a displacement change becomes large. Accordingly, if a movable portion comes into contact with a stationary mechanism such as a stopper or the like to fail to reach a target displacement, a large drive control value is maintained and a large current continues to flow.
As a result of this, the shape memory alloy continues to be heated and an excessive stress is generated. There are problems that because of this overheating and excessive stress, characteristics of the shape memory alloy deteriorate and part of the shape memory alloy actuator breaks down.
In connection with the above problems, a patent document 1 discloses an actuator that includes a sensor portion that detects an excessive stress which exceeds a predetermined threshold value is used for a shaft-direction load that acts on a shape memory member.    [patent document 1]: JP-A-2006-189045